Structure of a field effect transistor having metallic silicide and manufacturing method thereof

ABSTRACT

A field effect transistor having metallic silicide layers is formed in a semiconductor layer on an insulating layer of an SOI substrate. The metallic silicide layers are composed of refractory metal and silicon. The metallic silicide layers extend to bottom surfaces of a source and a drain regions. A ratio of the metal to the silicon in the metallic silicide layers is X to Y. A ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic silicides is X0 to Y0. X, Y, X0 and Y0 satisfy the following inequity: (X/Y)&gt;(X0/Y0).

[0001] The present application claiming priority under 35 U.S.C. 119 to Japanese Application No. 2000-104733 filed on Apr. 6, 2000, which is hereby incorporated by reference in its entirely for all purposes.

BACKGROUND OF THE INVENTION

[0002] (1). Field of the Invention

[0003] The present invention relates to a field effect transistor (FET) which is formed in a silicon layer located on an insulating film, or a silicon on insulator (SOI) substrate. The substrate has an insulating film and a thin silicon layer formed over a conductive substrate is used as the conventional substrate to form the FET, and to a method of manufacturing such field effect transistor.

[0004] (2). Description of the Related Art

[0005] Recently, a field effect transistor is formed on an SOI substrate (SOI-FET) in stead of a conventional bulk semiconductor substrate. The SOI-FET is formed in a thin silicon film (SOI layer) formed on an insulating film. As a junction capacitance is reduced, the SOI-FET can operate with a high-speed. Particularly, a fully depleted SOI-FET formed in the SOI layer is known as a low-power device which has a small parasitic capacitance and a subthreshold swing smaller than that of the conventional bulk semiconductor substrate. The fully depleted transistor has a depletion layer, which expands to the bottom surfaces of a source region and a drain region, when a voltage is supplied to a gate electrode thereof. As an expansion of depletion layers of the fully depleted SOI-FET is defined by a thickness of the SOI layer, a short channel effect can be restrained. Thus, for achieving the fully depleted operation of the SOI-FET, it is necessary to reduce the thickness of the SOI layer, as a device becomes microscopic.

[0006] The low-power device is shown in The article magazine of Institute of Electronics, Information and Communication Engineers C-II Vol. J81-C-II No. 3, pp. 313-319, “Deep Sub-0.1 μm MOSFET's with Very Thin SOI Layer for Ultra-Low Power Applications”, published in March 1998 (hereinafter a first thesis). The shorter the gate length of the SOI-FET, the thinner the thickness of the SOI layer is. In the case where the gate length is 0.1 μm, the thickness of the SOI layer should be set 20 nm or below. Therefore, it is necessary to reduce the thickness of the SOI layer, if the SOI-FET becomes smaller. The thickness of the SOI layer becomes thinner, the deterioration of the drive current occurs since the enlargement of the parasitic resistance in the source and drain regions becomes remarkable. Nowadays, a silicide technology using titanium (Ti) or cobalt (Co) is adopted to restrain the deterioration of the drive current. A metallic suicide layer is comprised of refractory metal and silicon. Titanium disilicide (TiSi₂) and Cobalt disilicide (CoSi₂) are widely used for their low resistivity.

[0007] However, when the specific contact resistivity between a metallic silicide layer and a silicon layer exceeds 1×10⁻⁷ Ω-cm², and the thickness of the SOI layer located under the metallic silicide layer is less than 10 nm, the parasitic resistance would be increased so as to increase a resistance of a diffusion layer. Asa result, the drive current of the SOI-FET might be reducing. This relationship is shown in IEEE ELECTRON DEVICE LETTERS, VOL. 15 No. 9, and pp. 363-365, “Optimization of Series Resistance in Sub-0.2 μm SOI MOSFET's”, published in September 1998 (hereinafter a second thesis).

[0008] Currently, no metallic materials which have a specific contact resistivity less than 1×10⁻⁷ Ω-cm² have been known. Hence, it is required that the SOI-FET layer be comprised of a silicide layer and a non-silicide layer over 10 nm in a diffusion layer. However, when a gate length becomes smaller, the thickness of the SOI layer become too thin to form such silicide and silicon multi-layered structure.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a field effect transistor, having a satisfied drive current thereof for adapting microscopic devices.

[0010] In order to achieve the above object, in a field effect transistor having metallic silicide layers composed of refractory metal and silicon. Wherein the bottom surfaces of the metallic silicide layers respectively extend to the bottom surfaces of a semiconductor layer. Wherein a ratio of the metal to the silicon in the metallic silicide layers is X to Y, a ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic silicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0).

[0011] According to the present invention, a field effect transistor capable of restraining the reduction of the drive current thereof. The transistor of the present invention can be formed in a microscopic size.

[0012] Typical ones of various inventions of the present application have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:

[0014]FIG. 1 is a cross-sectional view describing a field effect transistor according to a first preferred embodiment,;

[0015]FIG. 2 is a cross-sectional view describing a field effect transistor according to a second preferred embodiment,;

[0016]FIG. 3(a)-FIG. 3(b) are explanation diagrams describing relationship between the drain current and drain voltage characteristics of the fully depleted SOI-FET,;

[0017]FIG. 4 is an explanation diagram showing the drain voltage dependence of a threshold voltage for the fully depleted SOI-FET,;

[0018]FIG. 5(a)-FIG. 5(c) are cross-sectional views describing a method of fabricating a field effect transistor according to a third preferred embodiment,; and

[0019]FIG. 6 is an enlarged cross-sectional view illustrating the first preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. The drawings used for this description typically illustrate major characteristic parts in order that the present invention will be easily understood. In this description, one embodiment is shown in which the present invention is applied to a field effect transistor having metallic silicide layers.

[0021] First Embodiment

[0022]FIG. 1 is a cross-sectional view describing a field effect transistor according to a first preferred embodiment, for detail, typicality showing a fully depleted SOI-FET formed on an SOI substrate. The SOI-FET is formed in a thin silicon film (SOI layer) which is formed on an insulating film of the SOI substrate.

[0023] An SOI-FET 10 is formed on an SOI substrate which is comprised of a silicon substrate 1, an insulating layer 2 formed on the silicon substrate 1, and a thin silicon layer (SOI layer) 3 formed on the insulating layer 2. The SOI-FET 10 is formed in an active area surrounded by field oxide films 6 a and 6 b, and includes a gate electrode 5 of the SOI-FET is formed on a gate oxide film 4 which is formed on the SOI layer 3. A pair of highly doped silicon layers 8 a and 8 b is formed in a source region and a drain region under sidewalls 7 a and 7 b, respectively. A channel region is defined between the pair of highly doped silicon layers 8 a and 8 b.

[0024] The source and the drain regions excepting the highly-doped silicon impurity layers 8 a and 8 b are comprised of metallic silicide layers 9 a and 9 b. The metallic silicide layers 9 a and 9 b are composed of refractory metal and silicon. An amount of refractory metal contained in the metallic silicide layers 9 a and 9 b is more than that of silicon. In the first preferred embodiment, the metallic silicide layers 9 a and 9 b are comprised of a CoSiz layer in which a ratio of cobalt to silicon is one to z (1<z<2). In other words, a ratio of metal to silicon in the metallic silicide layer is X to Y, a ratio of metal to silicon of metallic silicide having the lowest resistance among stoichiometaric metallic suicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0). The CoSiz layers 9 a and 9 b are formed by a conventional silicide process, for more detail, all of the source region and the drain region except under the sidewalls 7 a and 7 b are changed into the cobalt silicide layers 9 a and 9 b. That is, bottom surfaces of the cobalt silicide layers 9 a and 9 b extend to bottom surfaces of the SOI layer 3. An electrical connection between the CoSiz layers 9 a and 9 b and the SOI layer 3 is performed between the highly doped silicon layers 8 a and 8 b and the CoSiz layers 9 a and 9 b only.

[0025] According to the second thesis as described above, in the case where the contact specific resistance between refractory metal and silicon becomes less than 1×10⁻⁷ Ω-cm², the parasitic resistance of the transistor does not increase and the drive current of the transistor does not reduce. Inventors of the present invention found that the metallic silicide layers 9 a and 9 b are able to cause the contact specific resistance between refractory metal and silicon to be less than 1×10⁻⁷ Ω-cm². A result of verification will be shown as follows.

[0026]FIG. 3(a)-FIG. 3(b) are explanation diagrams describing relationship between the drain current and the drain voltage characteristic of the fully depleted SOI-FET. In the first preferred embodiment, the SOI-FET 10 is an N type MOS transistor. A characteristic between a drain current and a drain voltage of a conventional SOI-FET is shown in FIG. 3(a). The conventional SOI-FET is comprised of an SOI layer having a thickness of 50 nm and a pair of impurity layers having a thickness of more than 10 nm under CoSi₂ layers. A characteristic between the drain current and the drain voltage of the first preferred embodiment is shown in FIG. 3(b). The SOI-FET 10 is comprised of an SOI layer 3 having a thickness of 20 nm, sidewalls 7 a and 7 b having a width of 0.1 μm, a pair of highly doped silicon layers 8 a and 8 b, and the CoSiz layers 9 a and 9 b formed by a silicide process. The CoSiz layers 9 a, 9 b are composed that all of the source region and the drain region except under sidewalls 7 a and 7 b are changed into the cobalt silicide layers 9 a and 9 b. Both of the conventional FET and the SOI-FET of the first preferred of the invention are comprised of a gate length of 0.18 μm, a gate width of 10 μm and a gate oxide film having a thickness of 4.5 nm.

[0027] The drain current shown in FIG. 3(a) is similar to the current shown in FIG. 3(b). Wherefore, it is easily understood that the drive current of SOI-FET structure in the first preferred embodiment becomes stable. In other words, the drain current of both structures becomes stable when the drain voltage thereof exceeds a predetermined voltage. Additionally, it has been already identified that the drive current of a P type MOS SOI-FET is as stable as that of the N type MOS SOI-FET comprised of the N type MOS transistor. A range of the total series parasitic resistance of the SOI-FETs of the present embodiment is obtained as follows. The total series parasitic resistance of the N type MOS transistor is about 400 Ω-μm. That of the P type MOS transistor is about 1K Ω-μm.

[0028] An estimated total series parasitic resistance of the SOI-FET of the first preferred embodiment will next be described. FIG. 6 is an enlarged partial cross-sectional view of the SOI-FET described in FIG. 1. Incidentally, the same reference numbers identifies the same elements of structure as the SOI-FET shown in FIG. 1 and the description of certain common elements will therefore be omitted.

[0029] The total series parasitic resistance Rtot of the SOI-FET 10 is derived from a following formula.

Rtot=2×(Rac+Rsp+Rsh+Rsh-s+Rco)

[0030] Wherein:

[0031] Rac represents an accumulation resistance of the highly doped silicon layer 8.

[0032] Rsp represents a spreading resistance of the highly doped silicon layer 8.

[0033] Rsh represents a diffusion layer resistance of the highly doped silicon layer 8 under a sidewall 7.

[0034] Rsh-s represents a sheet resistance of the CoSiz layer 9.

[0035] Rco represents a contact resistance between the highly doped silicon layer 8 and the CoSiz layer 9.

[0036] As the accumulation resistance Rac, the spreading resistance Rsp, and the sheet resistance Rsh-s are negligible small, they are out of consideration in the above formula.

[0037] If the impurity of 1×10²¹/cm³ is distributed in an N type drain region, the diffusion layer resistance of the N type MOS transistor becomes 80 Ω/sq. Also, if the impurity of 1×10²⁰/cm³ is distributed in a P type drain region, the diffusion layer resistance of the P tape MOS transistor is 500 Ω/sq. The diffusion layer resistances Rsh of the N type MOS and the P type MOS transistors each having a channel width of 1 μm are 192 Ω/μm and 450 Ωμm, respectively. The contact specific resistances between the N⁺ and P⁺ type drain regions and the CoSiz 9 is about 4×10⁻⁸ Ω-cm² and 9×10⁻⁸ Ω-cm², respectively.

[0038] In fact, it will be expected that the contact specific resistance go further reduce, by considering the accumulation resistance Rac, the spreading resistance Rsp and the sheet resistance Rsh-s in the above formula. The contact specific resistance between the cobalt silicide (CoSix) and the silicon of the first preferred embodiment is far smaller than 1×10⁻⁷ Ω-cm² introduced in the above second thesis.

[0039]FIG. 4 is an explanation diagram showing the drain voltage dependence of a threshold voltage for the fully depleted SOI-FET. In FIG. 4, the SOI-FET is the N type transistor, as well as FIG. 3(a) and FIG. 3(b). The drain voltage of the conventional SOI-FET is shown as a mark of “X” and that of the first preferred embodiment is shown as a mark of “”. When a substrate floating effect, such as a parasitic bipolar effect occurs in the fully depleted SOI-FET, the drain voltage will rise up. FIG. 4 shows that a reduction of a threshold voltage of the conventional SOI-FET is remarkable, as a drain voltage rises up. However, the reduction of the threshold voltage of the SOI-FET of the first preferred embodiment is loose. In other words, a leakage current of the SOI-FET can be cut down.

[0040] According to the first preferred embodiment of the present invention, a field effect transistor capable of restraining the reduction of the drive current of the SOI-FET. Further, the SOI-FET of the first preferred enables to form microscopic devices. Additionally, since the SOI-FET of the first preferred embodiment can precisely reduce the leakage current, the SOI-FET is useful as a low-power transistor.

[0041] Second Embodiment

[0042]FIG. 2 is a cross-sectional view describing a field effect transistor (FET) according to a second preferred embodiment. Particularly, the FET is a fully depleted SOI-FET formed on an SOI substrate. Incidentally, the same reference numbers identifies the same elements of structure as those shown in FIG. 1.

[0043] An SOI-FET 20 is formed on an SOI substrate which comprises a silicon substrate 1, an insulating layer 2 formed on the silicon substrate 1, and a thin silicon layer (SOI layer) 3 formed on the insulating layer 2. The SOI-FET 20 is formed in an active area surrounded by field oxide film 6 a and 6 b. the SOI-FET 20 has a gate electrode 5 formed on a gate oxide film 4 located on the SOI layer 3. A pair of highly doped silicon layers 18 a and 18 b is formed in a source region and a drain region. A channel region is defined between to the pair of highly doped silicon layers 18 a and 18 b.

[0044] The source and the drain regions include the highly doped silicon layers 18 a and 18 b and metallic silicide layers 19 a and 19 b, respectively. The metallic silicide layers 19 a and 19 b are composed of refractory metal and silicon. An amount of refractory metal contained in the metallic silicide layers 19 a and 19 b is more than that of silicon. In the second embodiment as well as the first embodiment, the metallic silicide layers 19 a and 19 b are comprised of a CoSiz layer having a ratio of cobalt to silicon is one to z (1<z<2). The CoSix layers 19 a and 19 b are formed by the conventional silicide process. For more detail, the source region and the drain region expect under the sidewalls 7 a and 7 b are changed into the cobalt silicide layers 19 a and 19 b.

[0045] The CoSiz layers 19 a and 19 b respectively have a thickness which is equal to or more than 80% thickness of from top surfaces of the CoSiz layers 19 a and 19 b to bottom surfaces of the SOI layer 3. In other words, portions 21 a and 21 b of the highly doped silicon layers 18 a and 18 b respectively extend between bottom surfaces of the CoSix layers 19 a and 19 b and a top surface of the insulating layer 2.

[0046] A conventional CoSi₂ layer is used as the metallic silicide layer. The conventional CoSi₂ layer is formed so as to have a thickness of the CoSi₂ layer less than 80% thickness of from top surfaces of the CoSi₂ to bottom surfaces of the SOI layer.

[0047] On the other hand, 1993 IEEE, pp. IEDM 93-723˜726-IEDM 93, “OPTIMIZATIN OF SERIES RESISTANCE IN SUB-0.2 m SOI MOSFETs” (hereinafter a third thesis), reports as follows.

[0048] 1) A void would be generated at an interface between a metallic silicide layer and a silicon layer.

[0049] 2) A parasitic resistance, which increases traceable to increase a sheet resistance of the metallic silicide layer. The void and the parasitic resistance influence a characteristic of a semiconductor device.

[0050] According to the second preferred embodiment of the present invention, since the metallic silicide layers are comprised of the CoSiz layer in which a ratio of cobalt to silicon is one to z (1<z<2), a field effect transistor can be formed by the CoSiz layer having a thickness which occupies equal to or more than 80% thickness of from a top surface of the CoSiz layer to bottom surfaces of the SOI layer. In fact, the degradation of a device characteristic traceable to above 1) and 2) can be perfected.

[0051] According to the second preferred embodiment of the present invention, as a small amount of silicon remains under the metallic silicide layers, a field effect transistor having a low resistance and a stable metallic silicide layer can be provided.

[0052] Third Embodiment

[0053] A method of manufacturing the field effect transistor described in the third preferred embodiment, will be shown hereinafter referring to FIG. 5(a)-FIG. 5(c). In FIG. 5(a), the SOI-FET is formed on an SOI substrate, which comprises a silicon substrate 51, an insulating film 52, and an SOI layer 53. Field oxide films 56 a and 56 b and heavily impurity layers 58 a and 58 b are formed in the SOI layer 53 by using a conventional process. A channel region is defined between the pair of highly doped silicon layers 58 a and 58 b. A gate electrode 55 is formed on a gate oxide film 54 and located on the channel region. Cobalt (Co) film 60 is formed on the gate electrode 55 and highly doped silicon layers 58 a and 58 b. The cobalt film 60 is a refractory metal film for forming a metallic suicide layer. A titanium (Ti) film 61 or a titanium nitride (TiN) film 61 are formed on the cobalt film 60 as an antioxidant film 61 when a following heat treatment.

[0054] Then, a first heat treatment is carried out. The first heat treatment is conducted at a temperature within a range of 400° C.˜600° C. with a lamp. Thereby, the cobalt silicide layers are formed on top surfaces of the gate electrode 55 and the highly doped silicon layers 58 a and 58 b. The cobalt silicide layers 62 a and 62 b are comprised of CoSi which has a composition ratio of cobalt to silicon that is 1:1, and Co₂Si that includes more cobalt than silicon. In the third preferred embodiment, the cobalt silicide layers 62 a and 62 b formed by the first heat treatment, are comprised of CozSi (z>=1) as stated previously.

[0055] After the first heat treatment, the antioxidant film 61 and the cobalt film 60 remaining on the field oxide film 56 and the sidewalls 57 a and 57 b, are selectively removed by a conventional etching agent, such as aqueous sulfuric or aqueous ammonia, as shown in FIG. 5(b). Thereby, the cobalt layer remaining on the gate electrode 55 and the highly doped silicide layers 58 a, 58 b are removed.

[0056] Then, a dielectric interlayer is formed over the SOI substrate including the cobalt silicide layer 62 a and 62 b. Contact holes are formed in the dielectric interlayer, by a conventional process. In the third preferred embodiment, an illustration and a description of the conventional process are omitted.

[0057] Then, a second heat treatment is carried out. The second heat treatment is conducted at a temperature equal to or more than 700° C. with a lamp. Thereby, cobalt silicide layers 63 a and 63 b are formed on top surfaces of the gate electrode 55 and all of the highly doped silicon layers 58 a and 58 b expect under sidewalls 57 a and 57 b, as shown in FIG. 5(c). The resistance of the cobalt silicide layers 63 a and 63 b are smaller than that of the CozSi layers 62 a and 62 b. In the third preferred embodiment, the cobalt silicide layers 63 a and 63 b formed by the second heat treatment, are comprised of a CoSiz in which a composition ratio of cobalt to silicon is one to z (1<z<2).

[0058] The third preferred embodiment could be applied to manufacture the SOI-FET explained in the second preferred embodiment, as well as the first preferred embodiment.

[0059] According to the present invention, a field effect transistor capable of restraining the reduction of the current drive capacity of the transistor. Also, the transistor enables to form microscopic devices including the SOI-FET described above. Since the transistor of the present invention can precisely reduce the leakage current, a low-power transistor can be provided. Additionally, as a small amount of silicon remains under the metallic silicide layers, a field effect transistor having a low resistance and a stable metallic silicide layer can be provided. 

What is claimed is:
 1. A field effect transistor including a gate electrode and a channel region defined by a source region and a drain region, comprising: an insulating layer; a semiconductor layer formed on the insulating layer, wherein the semiconductor layer includes the channel region therein; a pair of impurity layers formed in regions where are respectively contact with the channel region in the source region and the drain region; and a pair of metallic silicide layers respectively formed in the source region and the drain region, wherein the pair of metallic silicide layers are respectively contact with the pair of impurity layers, wherein bottom surfaces of the pair of metallic silicide layers extend to bottom surfaces of the semiconductor layer; wherein the metallic silicide layers are composed of refractory metal and silicon, and wherein a ratio of the metal to the silicon in the metallic silicide layers is X to Y, a ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic silicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0).
 2. The field effect transistor according to claim 1 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 3. A field effect transistor including a gate electrode and a channel region defined by a source region and a drain region, comprising: an insulating layer; a semiconductor layer formed on an insulating layer, wherein the semiconductor layer includes the channel region therein; a pair of impurity layers formed in regions where are respectively contact with the channel region in the source region and the drain region; and a pair of cobalt silicide layers respectively formed in the source region and the drain region, wherein the pair of cobalt silicide layers are respectively contact with the pair of impurity layers, wherein bottom surfaces of the pair of cobalt silicide layers extend to bottom surfaces of the semiconductor layer; wherein the cobalt silicide layers are composed of cobalt and silicon, wherein a ratio of cobalt to silicon is one to α(1<α<2).
 4. The field effect transistor according to claim 3 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 5. A field effect transistor including a gate electrode and a channel region defined by a source region and a drain region, comprising: an insulating layer; a semiconductor layer formed on the insulating layer, wherein the semiconductor layer includes the channel region defined by the source region and the drain region; a pair of impurity layers formed into regions where are respectively contact with the channel region in the source region and the drain region; and a pair of metallic silicide layers respectively formed in the source region and the drain region, wherein the pair of metallic silicide layers are respectively contact with the pair of impurity layers, wherein the pair of metallic silicide layers have a thickness which is equal to or more than 80% thickness of from the upper surface of the metallic silicide layers to the bottom surface of the semiconductor layer; wherein the metallic suicide layers are composed of refractory metal and silicon, and wherein a ratio of the metal to the silicon in the metallic silicide layers is X to Y, a ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic suicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0).
 6. The field effect transistor according to claim 5 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 7. A field effect transistor including a gate electrode and a channel region defined by a source region and a drain region, comprising: an insulating layer; a semiconductor layer formed on an insulating layer, wherein the semiconductor layer includes the channel region therein; a pair of impurity layers formed in regions where are respectively contact with the channel region in the source region and the drain region; and a pair of cobalt silicide layers respectively formed in the source region and the drain region, wherein the pair of cobalt silicide layers are respectively contact with the pair of impurity layers, wherein the pair of cobalt silicide layers has a thickness which is equal to or more than 80% thickness of from the upper surface of the cobalt silicide layers to the bottom surface of the semiconductor layer; wherein the cobalt silicide layers are composed of cobalt and silicon, wherein a ratio of cobalt to silicon is one to a (1<α<2).
 8. The field effect transistor according to claim 7 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 9. A field effect transistor formed in a semiconductor layer located on an insulating layer, the field effect transistor having a source region and a drain region formed in the semiconductor layer, comprising: the source region including a first impurity layer and a first metallic silicide layer, wherein the first impurity layer and the first metallic silicide layer are formed so as to reach the insulating layer through the semiconductor layer; and the drain region including a second impurity layer and a second metallic suicide layer, wherein the second impurity layer and the second metallic silicide layer are formed so as to reach the insulating layer through the semiconductor layer; wherein the first impurity layer is located so as to face to the second impurity layer, wherein a channel between the source region and the drain region is defined by the first impurity region and the second impurity, wherein the first metallic silicide layer and the second metallic silicide layer are composed of refractory metal and silicon, and wherein a ratio of the metal to the silicon in the metallic silicide layers is X to Y, a ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic silicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0).
 10. The field effect transistor according to claim 9 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 11. A field effect transistor formed in a semiconductor layer located on an insulating layer, the field effect transistor having a source region and a drain region formed in the semiconductor layer, comprising: the source region including a first impurity layer and a first cobalt silicide layer, wherein the first impurity layer and the first cobalt silicide layer are formed so as to reach the insulating layer through the semiconductor layer; and the drain region including a second impurity layer and a second cobalt silicide layer, wherein the second impurity layer and the second cobalt silicide layer are formed so as to reach the insulating layer through the semiconductor layer; wherein the first impurity layer is located so as to face to the second impurity layer, wherein a channel between the source region and the drain region is defined by the first impurity region and the second impurity, and wherein the first cobalt silicide layer and the second cobalt silicide layer are composed of cobalt and silicon, wherein a ratio of cobalt to silicon is one to α(1<α<2).
 12. The field effect transistor according to claim 11 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 13. A field effect transistor formed in a semiconductor layer located on an insulating layer, the field effect transistor having a source region and a drain region formed in the semiconductor layer, comprising: the source region including a first impurity layer and a first metallic suicide layer, wherein the first metallic silicide layer have a thickness which is equal to or more than 80% thickness of from the upper surface of the first metallic silicide layer to the bottom surface of the semiconductor layer; and the drain region including a second impurity layer and a second metallic suicide layer, wherein the second metallic silicide layer have a thickness which is equal to or more than 80% thickness of from the upper surface of the second metallic silicide layer to the bottom surface of the semiconductor layer; wherein the first impurity layer is located so as to face to the second impurity layer, wherein a channel between the source region and the drain region is defined by the first impurity region and the second impurity, wherein the first metallic silicide layer and the second metallic silicide layer are composed of refractory metal and silicon, and wherein a ratio of the metal to the silicon in the metallic silicide layers is X to Y, a ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic silicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0).
 14. The field effect transistor according to claim 13 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 15. A field effect transistor formed in a semiconductor layer located on an insulating layer, the field effect transistor having a source region and a drain region formed in the semiconductor layer, comprising: the source region including a first impurity layer and a first cobalt silicide layer, wherein the first cobalt silicide layer have a thickness which is equal to or more than 80% thickness of from the upper surface of the first cobalt silicide layer to the bottom surface of the semiconductor layer; and the drain region including a second impurity layer and a second cobalt silicide layer, wherein the second cobalt silicide layer have a thickness which is equal to or more than 80% thickness of from the upper surface of the second cobalt silicide layer to the bottom surface of the semiconductor layer; wherein the first impurity layer is located so as to face to the second impurity layer, wherein a channel between the source region and the drain region is defined by the first impurity region and the second impurity, and wherein the first cobalt silicide layer and the second cobalt silicide layer are composed of cobalt and silicon, wherein a ratio of cobalt to silicon is one to α(1<α<2).
 16. The field effect transistor according to claim 15 , wherein said field effect transistor has a depletion layer which expands to the bottom surfaces of the source region and the drain region when a voltage is supplied to the gate electrode thereof.
 17. A method for manufacturing a field effect transistor, comprising: preparing a substrate having a semiconductor layer formed on an insulating layer, wherein the semiconductor layer includes silicon; forming a gate electrode on a channel region in the semiconductor layer through a gate insulating film; forming insulating sidewalls on side surfaces of the gate electrode; forming a pair of impurity layers adjacent to the channel region in the semiconductor layer; forming a refractory metallic film on the substrate formed the gate electrode on the channel region in the semiconductor layer through the gate insulating film; exposing the refractory metallic film to a first heat treatment so as to form metallic suicide layers containing refractory metal and silicon; and exposing the first heat-treated metallic silicide layers to a second heat treatment; wherein a ratio of the metal to the silicon in the metallic silicide layers is X to Y, a ratio of the metal to the silicon of metallic silicide having the lowest resistance among stoichiometaric metallic silicides is X0 to Y0, and X, Y, X0 and Y0 satisfy the following inequity: (X/Y)>(X0/Y0).
 18. The method for the field effect transistor according to claim 17 , wherein the bottom surface of the second heat-treated metallic silicide layers reaches the upper surface of the insulating layer.
 19. The method for the field effect transistor according to claim 17 , wherein the second heat-treated metallic silicide layers have a thickness which is equal to or more 80% thickness of from the upper surface of the second heat-treated metallic silicide layers to the upper surface of the insulating layer.
 20. A method for manufacturing a field effect transistor, comprising: preparing a substrate having a semiconductor layer formed on an insulating layer, wherein the semiconductor layer includes silicon; forming a gate electrode on a channel region in the semiconductor layer through a gate insulating film; forming insulating sidewalls on side surfaces of the gate electrode; forming a pair of impurity layers adjacent to the channel region in the semiconductor layer; forming a cobalt film on the substrate formed the gate electrode on the channel region in the semiconductor layer through the gate insulating film; exposing the cobalt film to a first heat treatment so as to form cobalt silicide layers containing the cobalt and silicon; exposing the first heat-treated cobalt silicide layers to a second heat treatment, wherein a ratio of cobalt to silicon in the second heat-treated cobalt silicide layers satisfies one to α(1<α<2).
 21. The method for the field effect transistor according to claim 20 , wherein the bottom surface of the second heat-treated metallic silicide layers reaches the upper surface of the insulating layer.
 22. The method for the field effect transistor according to claim 20 , wherein the second heat-treated cobalt silicide layers have a thickness which is equal to or more 80% thickness of from the upper surface of the second heat-treated cobalt silicide layers to the upper surface of the insulating layer. 